The present invention relates to a load shedding system for shedding the loads of a building as the power consumed by those loads approaches a predetermined level and, more particularly, to a load shedding system in which those loads which affect a condition such as temperature and/or humidity in the spaces of a building are shed in the order of those loads associated with the space having the least deviation between the actual condition and a desired condition being shed first and those loads having the most deviation being shed last to maintain the average energy consumption rate below a predetermined level (i.e. current demand limit).
In recent years, with the renewed, increased interest in energy conservation, and particularly as a result of increases in the cost of fuels and a concomitant decrease in their availability it has become desirable and economically responsible to operate the various loads within a building in an energy conserving manner. Such buildings may, for example, comprise a plurality of floors each floor having both exterior and interior areas or offices (which may be referred to as zones or spaces) and each floor having one or more fan systems for supplying conditioned air to these zones. The conditions typically controlled by such systems are temperature and humidity.
Power companies have recognized the need for conserving fuel to keep down the cost of supplying power to their customers and to charge back to their customers an amount of money to cover the cost of extra equipment which is required to meet their customer's needs during those times of high energy demand. The capacity of the power companies' power generation plants is strained, particularly on those days having extreme temperatures. Extra or higher capacity equipment which normally is not fully utilized is required on such days in order for the power companies to meet the demands of all of their customers. In order to recover the cost of this extra capacity, power companies charge their customers not only for the power used during the billing period but also charge an extra amount based upon the highest average actual power consumption rate measured over a demand interval of time, usually selected to be 15 minutes, during the billing period. In order to minimize normal billing charges for power consumption, buildings have turned to such systems as load cycling for maintaining total power consumption during the billing period as low as possible and yet maintain a degree of comfort within the building. However, in order to minimize the extra charges resulting from high energy consumption rates, buildings use load shedding systems in which average power usage over a demand interval is monitored for shedding or turning off loads as the actual rate approaches a predetermined rate.
Heretofore, load shedding systems have either relied upon sequential shed tables or rotational shed tables. In sequential shed tables, the order of shedding remains fixed. That is, as the actual power consumption within a building approaches a given demand limit, load 1 is always shed first and load 2, if needed, is then shed next and so on. Restoration of loads is done in reverse order. Following restoration of all loads, subsequent shedding begins again with load 1. Conceivably, the loads at the end of the table may never be shed. In the rotational shed tables, on the other hand, the first time a shed routine is performed, load 1 through load n are shed first; but if the next action is an add (restore) action, load 1+n.+-.1 (if previously shed) will be first restored. If the next action is a shed action, load 1+n+1.+-.1 will be shed and so on. In any case, the order in which loads are shed in these two routines is not adaptive but is fixed at the time of either assembly or initialization. Only load deletions or reassignments at a later time affect the load shedding sequence. Consequently, unacceptable thermal or humidity discomfort often accompanies load shedding operations as the effects of sun, wind and internal latent heat within the building vary throughout the day or night.